1. Field of the Invention
This invention relates generally to a heat resisting aluminum alloy member reinforced locally by inorganic fibers, and a productive method of the same, and particularly but not exclusively is applicable to a piston head, a cylinder head and the like of an internal combustion engine.
2. Description of the Prior Art
In general, if a repetition of thermal loads is locally exerted on a member of a structure, cracks are initiated in the member due to repetition of local compression stresses at a higher temperature area and local tension stresses at cooling phase of the area, so that the life of the member is shortened.
For example, in an internal combustion engine, the repetition of thermal loads is exerted on an aluminum alloy piston head, the space between valves of an aluminum alloy cylinder head and the like, and it has been proposed already to reinforce locally such portions by metal matrix composite which contains fibrous inorganic reinforcing material, such as a SiC-whisker or a silicon nitride whisker, so as to prolong the life of the aluminum alloy piston, etc. (for example, see Japanese Laid Open Patent No. 62-233456).
However, the thermal expansion coefficient of the reinforced portion becomes very small compared with that of non-reinforced body portion, so that a difference of thermal expansion coefficient at an interface between the reinforced portion and the non-reinforced body portion of the member will cause a high stress at the interface at higher temperatures and finally cracks are initiated in the interface under the repetition of thermal loads.
One of effective means escaping from such damage is to enlarge the reinforced portion to keep the interface away from the hottest zone, lest the interface should be exposed to high temperatures, but according to such means, the amount of the expensive inorganic fiber increases to make the reinforced portion, and as a result, the cost of the heat resisting member will be raised.